1. Field of the Invention
The present invention relates to a scanning optical system, more particularly, to a scanning optical system with a high resolution which includes a plastic lens.
2. Description of the Related Art
A scanning optical system is essential to a laser printer, a laser scanner, a bar code reader or the like. In a scanning optical system, a polygonal mirror, a hologram disk or the like is used as a light deflector. A laser beam emitted from a semiconductor laser is incident upon, and deflected by, the light deflector. Then the laser beam passes through a scanning lens system such as an f.theta. lens system, to scan a predetermined area on a scanning surface (i.e., the main scanning is executed). The scanning surface is, for example, a sensitive paper or plate. While the main scanning is being executed, the scanning surface is moved in the direction perpendicular to the direction of the main scanning (i.e., the sub-scanning is executed). Thus, the scanning surface is two-dimensionally scanned.
The high precision f.theta. lens system used in a scanning optical system has conventionally consisted of more than two lenses made of glass including an anamorphic lens, and therefore it is costly to produce such a system. Japanese Unexamined Patent Publication No. 4-110817 has proposed an f.theta. lens system consisting of two toric lenses so as to make the f.theta. lens system totally out of plastics. A plastic lens is usually subject to variations in temperature and humidity. Thus, a deviation of the focal point in the f.theta. lens system will occur in the case where the f.theta. lens system is totally made of plastics. Japanese Unexamined Patent Publication No. 4-277715 has proposed an f.theta. lens system made totally of plastics, but is subject to a variation in temperature. However, a back focal length of this f.theta. lens system is short, which would restrict the construction of the system.